In a compressor operating in distinct phases, such as a piston compressor or a rotary compressor of a helical type, generally known as SRM, Lysholm, twin-screw or Globoid compressors, gas pulsations/pressure pulses will occur on both the low-pressure and the high-pressure sides. Usually these pulsations are strongest on the high-pressure side. The pulsations influence both the compressor itself and the pipes and other equipment connected thereto. The pulsations also affect the foundation and building where the compressor is located. This causes oscillations in the entire construction, which result in vibration and noise. Resonance oscillations may even occur, which will actually damage the construction.
In a closed-circuit rotary compressor the drive motor is integrated with the compressor and the operating medium flows over it. The motor may be located on the high-pressure side, that is after the compressor in the flow direction of the operating medium, in which case it will be direcly subjected to the gas pulsations mentioned above. The motor windings are mechanically relatively weak and are influenced by a pulsating magnetic field according to known patterns, as well as by the gas pulsations. The motor is housed in a relatively large casing, which is directly influenced by the pulsations that are superimposed on the high pressure. The housing may thus easily start vibrating.
Some compressors are run at different speeds, determined by the gear ratio, the pole number of the motor and the frequency of the power supply. It is thus extremely difficult to design the various elements, pressure vessel, motor windings, and so on, with resonance frequencies outside what can be considered as a risk area.